So-called photon-counting or spectral x-ray devices allow for detecting single x-ray photons hitting the radiation detector separately and for determining the energies of the incident photons in accordance with plural energy bands or “bins”. However, when two or more photons arrive at the radiation detector within a time interval that is shorter than the so-called deadtime of the detector, they are registered as a single event with a very high (wrong) energy. This effect, which is also known as pile-up effect, results in lost counts and in a distorted energy spectrum.
Also in view of the pile-up effect, the intensity of the x-ray beam may be selected such that the photon flux rate in the beam arriving at the detector after having been attenuated by the object to be imaged is not too high so that pile-up is avoided. However, also after a corresponding adaptation of the beam intensity pile-up does usually still occur in regions of the radiation detector collecting parts of the x-ray beam which are not or only weakly attenuated. This is particularly the case for parts of the beam which travel next to (and not through) the object to be imaged or which traverse only weakly attenuating regions of the object, such as, for example, lung tissue in case the object to be imaged is a human or animal body.
In order to reduce the pile-up effect for parts of the radiation detector receiving radiation directly from the x-ray source, beam-shaping filters—so-called bowtie filters—have been used for reducing the intensity of the parts of the radiation beam which do not traverse the object. However, in CT the relevant contour of the object varies, when the x-ray source and the radiation detector rotate around the object. Therefore, dynamic beam-shaping filters have to be used which allow for varying the beam shape during the rotation of the x-ray source and the radiation detector. Such dynamic filters are mechanically very complex and, thus, very expensive. Moreover, it is nearly impossible to attenuate the center of the x-ray beam using a beam-shaping filter as it would be necessary for imaging human legs, for example, or weakly attenuating regions within an object.
US 2011/0017918 A1 relates to a radiation detector that is suited for energy resolved single x-ray photon detection in a CT scanner. The detector comprises an array of scintillator elements in which incident x-ray photons are converted into bursts of optical photons. Pixels associated with the scintillator elements determine the number of optical photons they receive within predetermined acquisition intervals. Detector cells of the radiation detector can be designed such that they change from a sensitive state to an insensitive state upon detection of a single optical photon, and the detector cells are reset to the sensitive state during a reset interval. The duty cycle including the acquisition time and the reset time can be adapted to the detected flux. Thus, the pile-up effect may be reduced. However, upon having changed to the insensitive state, a detector cell can no longer detect photons until it is reset.
EP 2 371 288 A1 relates an imaging system comprising a radiation source and a detector. The system determines parameters associated with the radiation source and the detector based on a priori information and preliminary image data. With respect to the detector, the system may switch detector elements between a photon-counting and an energy-integrating mode based on the flux determined using the preliminary image data. US 2015/0063527 is related to an x-ray system comprising a photon-counting detector array of detector pixels that include direct conversion material. In order to determine the correct input photon count rate, multiple input photon count rates are mapped to a single output photon count rate. Then, the image data are reconstructed on the basis of the determined input photon count rate.